Add C wire


  #1  
Old 12-31-20, 09:09 AM
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Add C wire

I've read dozens of C wire posts, but I can't quite find the answer to my set up.

I have a crown boiler with 3 old fashioned mercury tsats. I want to replace one with a nest. The wire runnimg to the tstat is red, white, green, blue, but only the red and white are connected. At the boiler side, they go into a rats' nest of wires and connections, but the ble and green are just wrapped around the wire casing.

I have a transformer attached to the main shut off switch with red and white connected back to the rats nest. The white wire on the transformer says C. It's 24vac 40va.

Can I just attach the used blue wire runnimg to the tstat to the C terminal of that transformer and use that as C to power my Nest thermostat?

Alternatively, if I got a 24v C wire p/s could In use the unused blue and green somehow? That seems like the way to go, but the nest only has 1 R terminal, so I'm not sure about doubling the green and red wires to the R of the nest.

Thanks in advance.






Wires running to 3 old mercury tstats with spare blue and green wires.


Label on transformer to main cut off switch
 

Last edited by PJmax; 12-31-20 at 03:23 PM. Reason: cropped/resized/reoriented pictures
  #2  
Old 12-31-20, 10:05 AM
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Yes use blue wire for C. Thermostats are two wire switch that when contacts close starts heating.
New electronic TS require third wire for power to C terminal.

At basement unit where red and white go, find 24 volt source for C. Use volt meter to verify 24 volts to either red ore white and blue to it. If cannot find one get 120 to 24 volt AC adapter and connect wires to blue and red for TS RH (common color code).

At Nest use volt meter from blue to red and white. Measured from blue, one will be 24 volts and goes to RH. Other wire connect to W heat on most systems. Blue goes to Nest C.

DH likes idiot lights. To see from any place in room when thermostat is calling for heat have Red LED wired to C and W. (with 5 volt LEDs use 22,000 ohm resistor)
 

Last edited by doughess; 12-31-20 at 11:28 AM.
  #3  
Old 12-31-20, 03:17 PM
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Welcome to the forums.

I'm assuming you have three zones valves. Post a picture or two of the wiring on them.
The transformer labeling cannot be followed yet.
 
  #4  
Old 01-04-21, 07:35 AM
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Zone valves

Hi PJ,

Yes, I have 3 zones. Attached are images of the zoe valves, but due to their placement, I didn't want to touch them. Can I use the transformer from the master shutof


f?
 
  #5  
Old 01-05-21, 06:38 PM
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Yes, use one of terminals on transformer. Question is which one.

Connect blue to one transformer terminal, then follow Post #2 third paragraph.

Depending on sensitivity of volt meter, Red or White may measure different volts. For blue, use terminal with higher voltage then connect that wire connect to RH.
 

Last edited by doughess; 01-05-21 at 07:34 PM.
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Old 01-05-21, 07:36 PM
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It's more than using the right terminal on the transformer. You need to confirm that the proper wiring is going to R and W on the stat. I need to see the splice point of the valves. Looking at the following diagram.... That is the valve using screw terminals. In your case we're just interested in the two yellow wires. One yellow wire must go directly to the stat. That would be the W wire. The other yellow may be combined with the others. That may be the C line and goes to the transformer. The other transformer would go to a common splice of all the thermostats and be R.

Single zone valve system


............................... 3 Zone Valve system.
 
 

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